As is well known to those skilled in the art, when a well pipe or liner is positioned within a well bore or casing and a drilling fluid is circulated in the annular space between the well liner and the well casing, the drilling fluid may assume a "channelling flow" within the annular space. In this channelling flow condition only a portion of the fluid within the annular space is flowing relative to the well liner and well casing and this flowing portion assumes and follows passages or channels formed within the remainder of the drilling fluid that is remaining relatively static. This formation of channels and creating channelling flow occurs as a result, at least in part, of the gelling properties of the drilling fluid or mud in that, if the drilling fluid is allowed to remain in the static condition for a period of time, it will gel and a substantial hydraulic or mechanical agitating force is needed to again return the entire column of drilling fluid to a flowing fluid state.
During many procedures and processes in the drilling and completion of a well, such as cementing a well liner in a well casing and well bore, this channelling flow is highly undesirable. For example, in a cementing operation if this channelling flow occurs, the cement slurry will follow these channels resulting in an incomplete displacement of the drilling fluid from the annular space. Under these circumstances the cement fill will be incomplete and probably inadequate to achieve the desired support and seal between the well liner and the well casing. It has been found that hydraulic forces may be developed for eliminating this channelling flow by using extremely high fluid pumping velocities, but the pressures and volumes required have made such a method impractical due to the number of pumps required at the well site. Additionally, the use of high pump pressures might affect the competency of some earth formations resulting in the breakdown of a weak formation with the subsequent loss of drilling mud and/or cement slurry into the formation. The mechanical agitation forces necessary to return the gelled drilling fluid to a fluid condition have been accomplished by the use of scratchers, turbulence generating devices, and the like on the exterior of the well pipe and then rotating or reciprocating the well pipe to agitate the drilling fluid and cement slurry.
Turbulence generating collars which also act as centralizers are well known to the art as shown in U.S. Pat. Nos. 2,312,600--2,602,512--and 3,072,195. In general, these devices are clamped onto the outside of a well casing being run into a well and then reciprocated and/or rotated therein by either moving the casing up and down or rotating it. This action causes these devices and the spring members carried thereby to scrape mud from the borehole wall. Additionally, some of these centralizers have outwardly extending blades or vanes at spaced intervals along the pipe so as to cause turbulence in the mud column as the blades or vanes are reciprocated or rotated.
The centralizer elements of these well known centralizers are in the form of bowed springs which contact the borehole wall as they extend between a pair of collars surrounding the pipe. The collars are generally arranged to rotate on the well pipe or casing or slide up and down on the surface of the pipe between a pair of stop members arranged to limit their movement. The main drawback with the presently known turbulence centralizers is that they may be clamped or pinned on the outside of a pipe string and that they are generally hinged in construction for ease of assembly on the pipe. While this type of construction of the centralizer is quite adequate when being used on the outside of a well casing, it often creates a problem when used on the outside of a well liner concentrically arranged within a well casing near the lower end thereof, and hung therein by means of a liner hanger engaging the inner surface of the well casing.
On more than one occasion, it has happened that when a well liner has been run several thousand feet down to the bottom of the well casing in the well, a spring type hinged centralizer that is clamped on the outside of the well liner below the hanger thereof will break loose and slide up the outside of the well liner and become lodged on the liner hanger in a manner such that the hanger becomes inoperative and cannot be set to engage the inner surface of the well casing as designed. In such instances, the entire string of pipe along with the liner has to be withdrawn from the well to fix the centralizer and/or the liner hanger. Additionally, the spring bows of this type of hanger are not sufficiently rigid in some instances and take up too much space outside the pipe so that it cannot be run through a well casing where the internal diameter thereof is only slightly greater than the outside diameter of the well liner.
The use of bi-directional vanes on the outside of a pipe string within a well which also serve to centralize the pipe string within the well are shown in U.S. Pat. No. 3,176,771. The vanes of this mud scraper are arranged on the pipe for rotational and transitional motion along its axis. Thus, the flow of fluid caused by pumping cement within the casing on which they are mounted will cause the vanes to rotate about the casing.